This invention relates to heating coil assemblies for electromagnetic induction cooking heater, and methods for manufacturing the heating coil assemblies, more particularly, to heating coil assemblies having larger coil winding pitch at the middle part between the outer and inner circumferential parts thereof and methods for manufacturing the same.
An electromagnetic induction cooking heater comprises a heating coil assembly. The coil assembly is generally manufactured as follows: As shown in FIGS. 11 and 12, a Litz wire 3 consisting of a number of separately insulated strands of enameled wires is wound spirally and secured to a coil supporting base 2 with varnish. More specifically, as shown in FIG. 5, the coil supporting base 2 and a coil winding frame 1 are fixedly mounted on the shaft of a coil winding machine in such a manner that a space for receiving the Litz wire 3 is formed therebetween, and then the Litz wire 3 is spirally wound in the space. The coil manufacturing method is advantageous in that the number of manufacturing steps is relatively small and the productivity is high.
However, in the heating coil assembly manufactured according to the above-described conventional coil manufacturing method, the Litz wire is uniformly and closely wounded as is apparent from FIG. 11, thus providing the following difficulty: When the heating coil is energized, the middle part of the heating coil, located between the inner and outer parts thereof becomes higher in magnetic flux density than any other parts. Therefore, when a cooking utensil to be heated, such as a pan, is placed over the heating coil, its part confronting the middle part of the heating coil is heated more than the other parts. This phenomenon may result in the difficulty that, especially in preparing food by baking, the food is nonuniformly baked.
In order to eliminate the difficulty, a method has been proposed in the art in which the coil winding pitch is made smaller towards the outer periphery of the coil (cf. Japanese Patent Application Laid-Open No. 56743/1981). However, the method is low in productivity and high in manufacturing cost, because it is necessary to prepare a plastic spacer for manufacture of each coil and the wire is wound on the spacer.